The primary purpose of this project is to examine the long-term sensory and cognitive effects of prenatal exposure to methylmercury (MeHg). Twenty-three macaques were exposed in utero to moderate levels of MeHg in the early 1980s and are now 14-16 years old. Last year we completed testing on auditory pure tone detection in these animals. The monkeys were trained to perform an operant task so that we could examine auditory thresholds at various frequencies. Previous studies have indicated that MeHg exposure during the postnatal period has an effect on auditory threshold. This is the first study of in utero exposure effects. Results to date have not indicated an effect of MeHg exposure on auditory thresholds. It appears that postnatal exposure may be critical for this effect.